The subject invention refers to a muffler with catalytic converter, in which at least one catalytic converter element is located in the muffler, so that an essential part of all exhaust gases from the engine are forced to pass through the element and there be converted into cleaned exhaust gases.
Catalytic mufflers for internal combustion engines are well-known since a very long time, and have mainly been intended for cars. For portable working tools, such as chain saws, they have been available on the market to a small extent since the end of the 1980""s. Demands for low weight, size and cost have contributed towards the fact that catalytic converter technology was put into practice considerably later within this field. The catalytic mufflers which have been used for portable working tools, generally included a catalytic converter element built-up of coated thin sheet metal strips, e.g. a pleated or corrugated metal strip could be rolled together with a plane strip into a cylindrical element. Both strips are coated with a catalytic layer and the exhaust gases are conducted through the axial cavities which are created between the strips and in this manner the exhaust gases are converted. This type of catalytic converter element is comparatively expensive at the same time as it is sensible to vibrations and it therefore requires an elaborately designed mounting in order to obtain an acceptable lifetime, seen from a pure mechanical point of view.
Catalytic converter elements composed of a thread-formed material are known for a long time. These catalytic converter elements are generally designed like plates or cylindrical elements of different lengths. They are usually made of a stainless steel wire material which has been crochetted into a plane sheet, which then has been folded a couple of times, or, rolled together into a homogeneous cylinder. DE 3024491 also describes some examples of elements where a sheet has been rolled up to be a tubular element. This tubular element is mounted along an inner diameter. The mufflers described were mainly intended for use in cars. As far as the applicant knows no muffler with a thread-formed catalytic converter element has reached the market before. Since these catalytic converter elements generally can be manufactured at a low cost compared with other types of catalytic converter elements, probably the lifetime of these elements has been considered as unsatisfactory from a mechanical point of view. For, the heat generation in a catalytic converter element is high, especially in elements intended for two-stroke engines. In such an element temperatures of overbearingly 1000 degrees centigrade can occur. In the tests made by the applicant of such a muffler the mounting as well as the design of the catalytic converter element have turned out to be utmost important in order to achieve an adequate lifetime.
DE 19514828 and DE 19643191 are showing examples of catalytic mufflers where a catalytic material with an extremely limited stability has been used. The catalytic material is a fibre material which is enclosed between close-meshed nets on both sides. Consequently, the catalytic material is not composed of a self-supporting body but is completely dependent on support from essentially all sides. In order to achieve enough durability the close-meshed nets must therefore lie close to each other meaning that the catalytic converter element is small in thickness. Naturally this means that the duration of the flow passing through the element is short. It will therefore be difficult to achieve a high conversion ratio in the catalytic converter at the same time as the total design of the conversion unit will be relatively complicated and expensive.
Purpose of the Invention
The purpose of the subject invention is to substantially reduce the above outlined problems.
The above mentioned purpose is achieved in that the catalytic muffler in accordance with the invention having the characteristics appearing from the appended claims.
The catalytic muffler according to the invention is thus essentially characterized in that the element is designed as an essentially self-supporting body made of catalytic material, which is hollow, or partly concave, and has inner and outer surfaces, e.g. the body is shaped as a circular or non-circular or possibly even as a narrowing sleeve, a dome-shaped or angular bowl-shaped body, and the element is, directly or via intermediary elements, mounted to a deviding part inside the muffler, such as a partition wall or an outlet or inlet pipe, and the mounting is arranged so that at least one end surface is kept fixed at the same time as the element is supported at the outer surface by at least one part while the inner surface is essentially free. The element is thus designed as an essentially self-supporting body made of catalytic material. It means that the element does not have to be encased on all sides but certain surfaces can be left free. Since the element is essentially self-supporting the surfaces which have to be provided with inlet or outlet openings can be made with considerably fewer and larger holes. This results in a more simple and efficient design at the same time as it enables saving of costs.
The shape as well as the mounting of the catalytic element are of great importance for its lifetime. A hollow or partly concave shape is especially advantageous. It could be a sleeve or a bowl-shaped body, and these are normally circular or dome-shaped, but could also have a polygonal angular form. All these shapes described have in common that the element can have a great form stability and that the wires in the element can run around the element, e.g. a sleeve-shaped element can be created from a crochetted tube-shaped sleeve in that the ends of the sleeve are being folded into themselves and the sleeve is being pressed in an axial direction between an inner and an outer tool. In this manner a cylindrical or conical or possibly an angular sleeve can be created. The element consists of a number of closed threads extending around the element and this in turn of course creates a very great stability, which is advantageous considering the very high temperatures the element is being exposed to.
The mounting of the catalytic converter element into the muffler is extremely important since it affects the stability as well as the cooling of the element. As for an element formed as a sleeve it has turned out to be especially advantageous to hold both ends fixed and support the element at its outer surface. Preferably the ends are kept fixed in that they are inserted into adapted depressions in the surrounding parts. This creates a stable mounting of the ends, which will stable the whole element. This is particularly true if the element has a relatively limited length. To support the element at the outer surface is advantageous. Since the element becomes warmer than its own housing it tends to expand against the outer surface and in this manner it will get an improved support. The outer surface is larger than the inner surface and hereby the cooling of the outer surface can be more effective. Furthermore, as a rule the untreated exhaust gases from the engine have better access to the outer surface and its enclosure than to the inner surface. This contributes highly to a better cooling of the outer surface. From many points of view it is thus advantageous to support the outer surface and left the inner surface essentially free. This reasoning is also valid for a bowl-shaped body. Such a body has only one end surface. Preferably this end surface is kept fixed while the outer surface is supported and the inner surface is essentially left free. These and other characteristic features and advantages will become more apparent from the detailed description of various embodiments with the support of the annexted drawing.